The present invention relates to a subtalar joint implant for replacing a natural subtalar joint existing between the talus and calcaneum bones of a patient. The present invention is directed in particular to a subtalar joint implant that is reliable and provides realistic emulation of the functions of a healthy natural subtalar joint.
Arthritis is a common chronic condition as people become older, and with an ageing population the healthcare costs associated with osteoarthritis will continue to rise. Joint replacement is a successful treatment for hip, knee, shoulder and ankle arthritis allowing restoration of function for patients. Such joint replacements are able to provide realistic emulation of the functions of the non-damaged joints.
End-Stage ankle and hindfoot arthritis is a major problem that has been shown to affect quality of life as much as end stage heart disease. At the time of writing it is estimated that up to 70,000 people are affected by subtalar joint arthritis in the UK alone. The commonest cause is posttraumatic, occurring in up to 16.9% of patients following a calcaneal fracture. Other non-traumatic causes include longstanding flat foot, tendon dysfunction and inflammatory arthritis. Aside from the use of painkillers, activity modification, and injections of steroids, there is only a single operative treatment at present—removal of the subtalar joints and fusion of the talus to the calcaneum (subtalar joint fusion). Fusion causes transmission of stresses to adjacent joints that might later fail. Although subtalar joint replacement has been attempted, it has not been successful, with early failure rates in the series of the first 100 of greater than 50% (Fournol S, Medical Chirurgie Pied 15: 67-71, 1999).
The creation of a subtalar joint replacement that is as functionally effective and reliable as the widely used hip and knee replacements is inhibited by the higher complexity of the subtalar joint and the lack of a consensus on a detailed understanding of the mechanics of the joint. The ankle complex comprises a number of bones articulating by joints between them. It is commonly thought that the majority of up and down movements (doriflexion/plantarflexion) take place at the true ankle joint, between the tibia and talus, whereas side to side balancing movements (inversion/eversion) are thought to take place at the subtalar joint, which is the joint between the talus and calcaneum (heel bone). However, it has proved difficult to construct a mechanical arrangement that emulates the function of the subtalar joint in a reliable and realistic manner.
CN201001789 and WO 2013/025194 A1 discloses examples of arrangements in which talar and calcaneal components of a subtalar joint prosthesis are configured to be slidable relative to each other in order to emulate function provided by a natural subtalar joint. The realism of the emulation is limited, however, because the sliding is not constrained in the same way as the movement allowed by the natural subtalar joint especially under load. Excessive and/or unnatural movement may lead to clinical instability and early failure of the prosthesis.